Without limiting the scope of the present invention, its background will be described with reference to conductive, filled elastomeric composites.
When particulate additives are mixed into polymeric systems at a sufficient concentration, they will touch each other. This formation of a continuous phase of particles within the polymeric system is known as percolation. If these additives are electrically conductive, they will impart conductivity to the polymeric system when added in concentrations greater than the percolation threshold concentration. When the additives are spherical, the electrical percolation threshold is theoretically achieved at a concentrations above approximately 16%. (S. Kirpatrick, Percolation and conduction, Rev Mod Phys 1973, 45 574-88).
Elastomeric compounds are materials that can undergo a large degree of elastic (i.e., full recovery) deformation. When elastomeric compounds are loaded with conductive additives to the point of percolation, they become conductive. However, because conductive additives are typically high stiffness, non-elastic particulates, when these systems are stretched, the percolating contact is lost. Accordingly, elastomeric systems have not previously been available where a high degree of conductivity persists in the stretched state.
While it is known in the art to add carbon nanotubes to polymeric systems in order to impart a variety of properties to the polymeric system, there have been no demonstrations of the retention of conductivity in the face of significant elastic deformation.
Slay et al., for example, in U.S. Pat. No. 7,696,275, disclose the use of carbon nanotubes in an elastomer as a mechanical reinforcement to improve the blow-out resistance of seals for oil well applications.
Howard, in U.S. Pat. No. 7,527,750, discloses a composition for sheet molding compounds that will impart conductivity to a molded component. While the conductivity of the filled systems in Howard could be substantial, his use of carbon black, which is nearly spherical, precludes persistent conduction when the composition is elastically stretched.
Anand et al., in U.S. Pat. No. 8,250,927, disclose a flexible, stretchable strain sensor material comprised of carbon nanotubes in an elastic matrix. Anand's described preparations achieve a gauge factor of 4 or greater by the admixing of nanotubes and carbon black in the elastic matrix, where the nanotube loading was <1% by weight. As will hereinafter be discussed, the utility of the present invention is to maintain conductivity despite the application of strain. To do so, the nanotube content in the present invention (see below) is typically greater than 3% by weight. Accordingly, the composites of the present invention have a gauge factor of less than 4. This is achieved in part by employing higher concentrations of carbon nanotubes in the present invention, but also by employing carbon nanotubes with significant lengths, e.g., >5 microns, with the present invention.
Noguchi et al., in U.S. Pat. No. 7,785,701, disclose a carbon nanofiber-filled elastomer material, having specific chemical interaction between the elastomer and the nanofiber. By contrast, and as will hereinafter be discussed in detail, with the present invention, no such specific chemical functionalization is required in order to achieve dispersion and conductivity, and as such, the present invention represents a significant advance in the state of the art.